Session: Advances in Spectroscopy:

Atropisomerism is a fundamental feature of substituted biaryls resulting from rotation around the biaryl axis. Different stereoisomers are formed due to restricted rotation about the single bond, a situation often found in substituted porphyrins. Previously NMR determination of porphyrin atropisomers proved difficult, if not almost impossible to accomplish, due to low resolution or unresolvable resonance signals that predominantly overlapped.

Here, we shed light on this fundamental issue found in porphyrinoid stereochemistry. First, we designed a lock–and–key comparable molecular receptor system for charge–assisted complexation. This allows the counter anion to lock in the anion-selective inner core system and reduce the proton exchange rate, generating static proton signals. While in complex form, porphyrin atropisomers stand in rotation–restricted states for which an ideal case study of atropisomers via high-resolution NMR is available. Next, by using benzenesulfonic acid (BSA) for host–guest interactions and performing 1D, 2D NMR spectroscopic analyses, we were able to characterize all four rotamers of the nonplanar 5,10,15,20–tetrakis(2–aminophenyl)–2,3,7,8,12,13,17,18–octaethylporphyrin. Lastly, upon detailed assignment of the chemical signals, symmetry operations, found unique to porphyrin atropisomers, offered an essential handle to accurately identify the rotamers using NMR techniques only. Overall, the N–H...X-type interplay in porphyrins formed by weak interactions that form restricted H–bonding complexes shows to be a key to unravel the atropisomeric enigma.

The selective encapsulation of analytes by nonplanar porphyrin atropisomers leads to the well restricted supramolecular ensemble. This results in a well–restricted system to issue an ideal tool for spectroscopic analysis and determinations of individual rotamers.

The selective encapsulation of analytes by nonplanar porphyrin atropisomers leads to the well restricted supramolecular ensemble. This results in a well–restricted system to issue an ideal tool for spectroscopic analysis and determinations of individual rotamers.

Prof. Dr. Mathias O. Senge

Chair of Organic Chemistry, Trinity College Dublin, The University of Dublin

Appearances